Solid members moving through or in contact with moving liquids are subject to stresses and strains. These conditions are aggrevated when the fluid flow is turbulent as opposed to laminer. With turbulent flow, substantial negative pressure at localized areas can develop which can result in cavitation. Voids or bubbles are formed in the liquid under such conditions which then collapse as positive pressure is returned. When the bubble collapse occurs on a solid surface it results in a large transfer of momentum from the liquid to a very small area of the solid. The collapse of these bubbles occurs with such force that solid members subjected to this force experience extreme localized stress, fatigue, corrosion, and erosion as well as pitting and flaking and, in extreme cases, outright rupture of the solid member.
Some well known examples of solid members that encounter liquids under the aforesaid conditions are pipes, pumps, propellers in water, the hull of a ship, the hydraulic drive on an automobile, hydraulic machinery of all kinds, the submerged members of a hydrofoil vessel, and water-driven turbines.
Surface coatings such as oxides, hydroxides, or carbonates form on some metallic surfaces in contact with water and protect the under metal from certain attack. However, these coatings do not form on all metals and not all liquids will cause them to form. Also, these protective coatings can be ruptured by a negative pressure on the surface as well as by the transient high positive pressure produced during the collapse of a cavitation bubble on the solid surface.
Additional deleterious effects on solid members caused by contact with turbulent fluids are:
1. The rupture of the liquid under high negative pressure, i.e. cavitation, can disrupt the molecules in the liquid thereby producing highly reactive free radicals and ions which can then chemically attack the solid member. PA1 2. Repeated alternating of high negative and positive pressure exerted on the solid members, particularly metalic members, cause them to flex and eventually fail through fatigue, crystalization or brittleness. PA1 3. Corrosion and erosion noted above increases the friction between the liquid and the solid member and hence lowers the overall efficiency of the system.
A primary method of coping with the various problems associated with turbulent fluid flow is to reduce the relative velocity between the solid member and the liquid. This, however, negates the very purpose that the system was constructed to achieve. As an example reducing the revolutions per minute (r.p.m.) of a propeller on a ship will reduce the deleterious effects of the turbulent liquid but only at the expense of reducing the ship's speed with its attendant additional costs.
A second method for coping with the deleterious effects of turbulent liquids is to replace the deteriorated solid members as they loose their effectiveness through corrosion and erosion. This is expensive, causes down time, and when the deterioration of the solid member is not detected in time, complete failure of the solid member can result.